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The present invention relates generally to millimeter wave radar, and more specifically to a millimeter wave radar system including a microstrip antenna array that provides reduced return loss at microstrip antenna ports.
In recent years, millimeter wave radar has been increasingly employed in automotive vehicles as part of Adaptive Cruise Control (ACC) systems. A conventional millimeter wave radar system adapted for ACC applications includes an antenna assembly such as a microstrip antenna array assembly that can be mounted on an automotive vehicle. The microstrip antenna array assembly is configured to transmit one or more directional beams to scan a field of view ahead of the vehicle, and receive one or more electromagnetic waves reflected from objects within the field of view to collect certain information about the objects. For example, the collected information may include data on the relative speed, direction, and/or distance of the objects in a roadway ahead of the vehicle. Further, the ACC system may use that information to decide whether to alert a driver of the vehicle to a particular obstacle in the roadway and/or automatically change the speed of the vehicle to prevent a collision with the obstacle.
The microstrip antenna array assembly included in the conventional millimeter wave radar system comprises a waveguide disposed on a surface of a backing plate, and a microstrip antenna array assembly operatively disposed on a surface of the waveguide. The waveguide includes a plurality of sections having slots formed therethrough such that junctions of the waveguide, the slots, and the microstrip antenna array define a plurality of respective waveguide-slot-microstrip transitions. The conventional millimeter wave radar system further includes a transmitter/receiver unit configured to transmit electromagnetic wave energy to the waveguide for subsequent transfer to the microstrip antenna array via the waveguide-slot-microstrip transitions, and receive electromagnetic wave energy from the waveguide via the microstrip antenna array and the waveguide-slot-microstrip transitions.
One drawback of the conventional millimeter wave radar system is that there is typically significant return loss at the respective waveguide-slot-microstrip transitions due primarily to impedance mismatches between the waveguide and the microstrip antenna array. Such losses can adversely affect the transmission of directional beams by making it harder to achieve full illumination of the microstrip antenna array. This is particularly problematic in ACC systems because it can compromise the validity of information collected on objects in a roadway ahead of a vehicle, and can lead to improper decision making regarding whether to alert a driver of the vehicle and/or automatically change the speed of the vehicle to prevent a collision with an obstacle in the roadway.
It would therefore be desirable to have a millimeter wave radar system that can be employed in automotive ACC applications. Such a millimeter wave radar system would include a microstrip antenna array assembly providing reduced return loss at waveguide-slot-microstrip transitions to enhance the performance of the overall system.
In accordance with the present invention, a millimeter wave radar system is disclosed that includes a microstrip antenna array providing reduced return loss at microstrip antenna ports. Benefits of the presently disclosed system are achieved by configuring the microstrip antenna array so that respective waveguide-slot-microstrip transitions at the microstrip antenna ports can more efficiently transfer electromagnetic wave energy between the microstrip antenna array and at least one waveguide included in the system.
In one embodiment, the millimeter wave radar system includes at least one channel formed in a metal backing plate and an adjacent microstrip antenna array assembly. The microstrip antenna array assembly includes a substantially planar circuit board, a single microstrip antenna array disposed on a first surface of the circuit board, and a ground plane disposed along a second circuit board surface such that a dielectric substrate of the circuit board is between the microstrip antenna array and the ground plane. The combination of the microstrip antenna array, the dielectric substrate, and the ground plane forms a plurality of microstrip transmission lines. Further, the ground plane is mounted to the metal backing plate comprising the at least one channel to form at least one waveguide. The ground plane has a plurality of slots formed therethrough along at least one line. The plurality of slots is transversely located relative to the microstrip transmission lines and longitudinally located relative to the waveguide, thereby forming a corresponding plurality of waveguide-slot-microstrip transitions for transferring electromagnetic wave energy between the microstrip transmission lines and the waveguide.
At least one open circuit stub is placed on each microstrip transmission line to match the impedance of the respective microstrip transmission line and the waveguide. The open circuit stubs are configured to add capacitive reactance to the respective microstrip transmission lines to cancel out a net inductive reactance at the waveguide-slot-microstrip transitions. In a preferred embodiment, the open circuit stubs are rectangular stubs positioned on the respective microstrip transmission lines so that each stub is in registration with a respective slot in the ground plane.
By employing capacitive stub matching on the microstrip antenna array to cancel out the net inductive reactance at the waveguide-slot-microstrip transitions, return loss is reduced at the microstrip antenna ports of the millimeter wave radar system. As a result, full illumination of the microstrip antenna array can be achieved, thereby making it easier to transmit a plurality of directional beams using the single microstrip antenna array.
Other features, functions, and aspects of the invention will be evident from the Detailed Description of the Invention that follows.